A surprise at the bottom of the main sequence: Rapid rotation and NO H-alpha emission

摘要: We report Kech Observatory high-resolution echelle spectra from 640-850 nm for eight stars near the faint end of main sequence. These are highest resolution such late-type stars, and clearly resolve TiO, VO, atomic lines. The sample includes field brown-dwarf candidate, BRI 0021-0214 (M9.5+). Very unexpectedly, it shows most rapid rotation in entire samples, v sin i approximately 40 km/s, which is 20x faster than typical nonemission M stars. Equally surprising that 0021 exhibits no emission or absorptionat H-alpha. argue this absence not simply due to its cool photosphere, but stellar activity declines a fundamental way at As first very late dwarf observed high spectral resolution, may be signaling qualitative change angular momentum loss rate among lowest mass Conventionally, would have marked as young, consistent with selection effect arises if latest-type dwarfs really brown on cooling curves. In any case, unprecedented find sign rapidly rotating convective star. also discuss possible conflict between observation extremely strong H-alpha seen another star, PC 0025+0447. Extrapolation M-L relations yields 0.065 solar mass, other objects expected masses H-burning limit. include two Pleiades candidates, four M6 one T Tauri 26 37 emission, radial velocities membership. Similarly, star has 30 km/s H alpha indicate youth. Two dMe exhibit above 5 expectations. measurable absoprtion lithium, indicating likely more massive mass.